Isoprene residues

In some cases, an isoprene residue is used as an element to modify molecules chemically. One example of this is N -isopentenyl-AMP, which occurs as a modified component in tRNA. 

Dolichols are long-chain poly(isoprenol)s found in eukaryotes only, and consist of 13-22 isoprene units. There are two internal, trails-iso-prene units, the other internal units being ct.s-isoprene residues.1... 

The chemical interrelationships among these seemingly widely different types of compounds justify the recognized segregation of the terpenes from other fields of organic chemistry. The structural significance of the isoprene residue ( C-C-C) to the different types of terpene carbon skeletons has already been indicated. The simplest type of terpene is an open-chain olefinic structure formed by the union of two isoprene units. Such compounds are called the acyclic terpenes. From two units of isoprene or from the simple acyclic structures, under proper conditions, simple... 

Polyterpenoids.—The presence of dolichols (202) with up to 23 isoprene units, in various sources, has been reported.In addition to these dolichols, which have their terminal isoprene unit saturated, fully unsaturated polyprenols have been found. Bovine pituitary contains a mixture of cis-trans-isomers of decaprenol (203), a Cioo prenol from bovine thyroid has been shown by n.m.r. to have eighteen cis- and two frans-isoprene residues, and 2,3-dehydrodoli-chyl pyrophosphate (204) has been identified as a product of a chick enzyme system and isopentenyl pyrophosphate.The structure of ulmoprenol from Eucommia ulmoides has been elucidated (207). ... 

TERPENES The terpenes are classified according to the number of isoprene residues they contain (Table 11.4). Monoterpenes are composed of two isoprene... 

Wellburn et al. (1967) have described the isolation and purification of a mixture of ci s-frans-polyprenols from the leaves of the horse chestnut tree. Results of studies involving mass, NMR, and infrared spectroscopy and various chemical reactions showed that each of the prenols contains three trans internal isoprene residues and a cis OH-terminal isoprene residue. They differ from each other only in the number of cis internal isoprene residues. The names castaprenol-11, -12, and -13 were given to these compounds. 

The basic skeleton of isoprenoids may be modified by the introduction of a wide variety of chemical groups, by isomerization, shift of double bonds, methyl groups, etc. Hence a bewildering number of chemical structures arises. In addition compounds derived from other biogenic pathways may contain isoprene residues. For instance the K vitamins (D 8.1), ubiquinones (D 8.3), chlorophylls (D 10.1), plastoquinones, and tocopherylquinones (D 22.4) have isoprenoid side chains with up to ten isoprene units. Polyketides (D 3.3), alkaloids (D 8.4.2), and coumarins (D 22.2.2) may be substituted by dimethylallyl groups. The terpene residues are attached to nucleophilic sites, such as active methylene groups and phenolic oxygen atoms. 

Diterpenes comprise four isoprene residues. They are either aliphatic substances or in most cases tri- or tetracyclic compounds, which may be substituted by different functional groups. Some diterpenes contain nitrogen (diterpene alkaloids, see the formula of aconitine). 

Naphthoquinones and anthraquinones are derivatives of the bicyclic and tricyclic aromatic compounds, naphthene and anthracene. The naphthoquinones of the vitamin K type possess a long isoprenoid side chain. Other naphthoquinones are devoid of a side chain, e.g., juglone (Fig. 144) or are substituted only by an isoprene residue, e.g., deoxylapachol. A typical representative of the anthraquinone derivatives is alizarin (Fig. 144). 

Prenyl-naphthohydroquinone-2-carboxyhc acid is the key intermediate in the formation of anthraquinones of the alizarin type. (In contrast the A-ring-substituted anthraquinone derivatives found in fungi and certain higher plants, e.g., Rhamnaceae and Polygonaceae, are polyketides, D 3.3.5). In the biosynthesis of alizarin one C-atom of the isoprene residue is lost. In structurally related compounds, however, this C-atom may still be present in the form of a methyl, hydroxymethyl, aldehyde, or carboxy group. 

Ubiquinones are prenylated benzoquinone derivatives carrying a side chain of 5-10 isoprene residues. Ubiquinone 50 (coenzyme Qio) for instance, has a side chain of fifty carbon atoms (=10 isoprene units). 

Another compound of interest is dolichol which was first isolated from liver and is now known to be present in many other animal tissues. It is a long chain molecule (n=17-21), its first isoprene residue is saturated and it has 2 trans double bonds. It is found in liver part as the free alcohol and another part esteri-fied with fatty acids. We now know that a small amount is present as the phosphate or pyrophosphate and may have various sugars joined to it. These dolichol derivatives are involved in the glycosylation of proteins. An excellent review on polyprenols has been written by Hemming (1974). 

Some of the reactions shown in.the scheme have also been detected in plants (Pont Lezica et al., 1975 Lehle et al., 1976 Brett Leloir, 1976). The polyprenol involved has been shown, by chromatography on Sephadex in deoxycholate solution to be smaller than liver in size, it has about 17 or 18 isoprene residues as compared to liver in which the compound with 19 residues is the most abundant. The compound is quite different from ficaprenol which is the major polyprenol found in plants and has 11 isoprene residues. The reactions detected in plants apparently include the formation of Dol-P-Man, Dol-P-Glc, Dol-P-P-acetylglucosamine and also some Dol-P-P-oligosaccharides. Since plants have glycoproteins of the same type found in animals it seems very likely that the scheme is also valid for them. 

Figure 1. Structure of polyprenols. - isoprene residue, t - number of internal trans residues, c - number of internal cis residues, - isoprene residue...

These result from the condensation of two (monoterpenes), three (sesquiterpenes), four (diterpenes), six (triterpenes), or eight (tetraterpenes) isoprene residues. The lower terpenes are widely distributed in the essential oils of many plants. Phytol, ah imsaturated alcohol present in the chlorophyll molecule, is a diterpene derivative. 

There are two classes of natural carotenoids the carotenoids proper, having a C-40 chain (8 isoprene residues), and the carotenoids possessing less than 40 carbon atoms. The natural carotenoids can be considered as related to lycopene (C40H5 ), the red pigment of the tomato. 

A short-hand convention to describe structures of polyprenols has been suggested (392). Abbreviations a> (co-isoprene residue), T (/A flrA25 -isoprene residue), C (c/5-isoprene residue), and S (saturated isoprene residue) are used to designate a structure. Thus, the all- / aA25 -nonaprenol, solanesol, the first polyprenol to be isolated from nature, can be represented as cu-T-T-[T]5-T-OH. 

It is remarkable that the biogenesis of the hop resins is almost complete before the formation of essential oil components starts (76,77). The isoprene residues are coupled to terpenes when an excess is present with respect to the biogenesis of the hop bitter acids. 

The true carotenoids and xanthophylls have 40 carbon atoms this corresponds to 8 isoprene residues. Like squalene, they are constructed symmetrically and it is assumed that they arise by head-to-head condensation of two C20 precursors. It has been found that mevalonic acid is one important precursor the appropriate enzyme systems from plants can convert mevalonate to carotenoids. The tetra-... 

Ki, phytyl side chain) two vitamins K with six and seven isoprene residues in the side chains (and six or seven double bonds) have also been discovered. The chain is built analogously to the condensation, forming farnesyl phosphate. In the bioassay, menadione (2-methylnaphthoquinone) is also active, presumably because it is converted to vitamin K2 in the organism (proof with C -menadione). 

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